This invention relates to a laser measuring device.
Measuring devices using laser beams are in common use. They are employed for various measurement purposes, e.g., to measure tank-contents level, to measure the distance between objects, and to scan surface structures, and are based on the principle of transit-time measurement. Laser pulses from a laser source are directed to an object, and light beams reflected from this object are received by evaluation electronics. The time required for a pulse to travel from the laser source to the object plus the travel time of the light beams reflected from the object to a detector is generally the measured quantity to be evaluated.
If the laser source and the detector are located at the same distance from the object, this distance is
d=tc/2
where t is the measured round-trip time, and c is the velocity of light.
By directing the laser beams to the surface of material in a tank, for example, and detecting the reflected light beams, changes in the level of the material can be detected and monitored.
The laser source sends the laser pulses through a protective pane or a lens in the housing of the laser measuring device, and the detector, which is preferably integrated with the laser source in the housing, receives the reflected light pulses through this protective pane or lens.
A problem associated with such laser measuring devices is the necessary continuous maintenance of the device, particularly if it is used in a dusty or otherwise contaminative environment. The protective pane or lens must then be cleaned prior to each measurement, so that the device can operate correctly and provide reliable data.
WO-A 95/10030 discloses a laser level measuring device whose housing has a lateral opening through which the protective pane or lens has to be regularly cleaned by hand, namely with a cleaning cloth.
It is an object of the invention to provide a substantially maintenance-free laser measuring device wherein dirt deposits on the lens or protective pane in the exit and entry window of the device are avoided as far as possible, so that cleaning, particularly prior to each measurement, is unnecessary.
To attain this object, the invention provides a laser measuring device
wherein in front of a window for the passage of laser beams
which contains a protective pane or lens,
a compressed-air chamber is provided
which has a compressed-air connection that is connected by a compressed-air line to a source of compressed air, and
to which compressed air is supplied in operation, and on whose side remote from the window there is provided an air lock
in whose open condition a light channel is open to permit the passage of the laser beams and of light beams reflected from an object,
in whose closed condition the light channel is closed, and
which is open only during a measurement.
The basic idea of the invention is to clean the protective pane or the lens of the measuring device with a jet of compressed air. A jet of compressed air permanently sweeping over the protective pane or lens, however, would entail considerable expense, inter alia because a compressor providing the compressed air would have to be in continuous operation. Therefore, a compressed-air chamber that is supplied with the compressed air via a compressed-air connection and a compressed-air line containing a control element is provided in front of the window for the laser and light beams.
In a first preferred embodiment of the invention, the compressed-air line contains a control element that blocks the passage of compressed air when the air lock is open.
In a second preferred embodiment of the invention, the control element permits the passage of compressed air when the air lock is open.
According to a first improvement of the invention and of the embodiments just mentioned, the control element sets or limits the pressure in the compressed-air chamber to a maximum of 200 kPa (=2 bars).
In a third preferred embodiment of the invention, the air lock is operated by means of a motor and the operation is synchronized with the beginning of a measurement.
According to a improvement of the third preferred embodiment of the invention, the motor is electrically or pneumatically driven.
In a fourth preferred embodiment of the invention, an oil filter is provided in the compressed-air line between the compressed-air chamber and the source of compressed air.
One advantage of the invention is that the overpressure created in the compressed-air chamber when the air lock is closed prevents dust or dirt particles from entering the compressed-air chamber and depositing on the protective pane or lens in the window. When the air lock is briefly opened for a measurement, air will escape from the compressed-air chamber and thus prevent dust or dirt particles from reaching the compressed-air chamber or the protective pane or lens during a measuring operation. This advantage also accrues in the second preferred embodiment of the invention.
The laser measuring device is thus substantially maintenance-free since cleaning of the window at regular, short time intervals as is required in the above-mentioned prior art is not necessary.
Since the air lock is opened only for the duration of a measurement, and the pressure in the compressed-air chamber in the closed condition of the air lock can be limited to a reasonable value, the overall consumption of compressed air is very low. The compressed air can be provided by a small pump during intervals between measurements. Costly compressed-air lines are not necessary.